This invention relates to composite insulators, especially for high-tension open-air use.
Two different constructional forms of insulator are already known. In one case the insulators are of the same material throughout and in the other case they have an internal part, which takes up the mechanical forces: this is fitted with external barriers or screens. The materials of the two elements are different and are so chosen as to suit the different functions of the two elements. In the latter case, the barriers or screens (which are insulating) when secured on the internal part, e.g. a synthetic plastic rod, serve to increase the creepage distance. This latter type of construction is known by the term "composite insulator".
High-tension composite insulators of synthetic plastic materials must conform to specific electrical requirements. The carrier rod must be electrically insulating in its axial direction and the insulating barriers or screens must be fitted in such a way that no electrical conduction can occur at the seam between the barriers or screens and the rod. Moreover, the barriers or screens must be so dimensioned that their thickness is sufficient to prevent their electrical resistance being overcome. Furthermore, the material of the barriers or screens must have not only good weather stability, ultra-violet stability and ozone stability but also an outstanding electrical tracking resistance.
For high-tension composite insulators, widely varying materials are known for the inner core and for the insulating barriers or screens fitted on it; by way of example, the barriers or screens may be produced from porcelain, glass, clay, stone material or even molded plastic material and hard paper may be used for the core. The insulators have been so designed that seals are provided between the barriers or screens themselves and also between the barriers or screens situated at the ends and any fittings, usually of metal, for attaching the insulator to a support and for attaching a conductor to the insulator. The seals are intended to prevent the penetration of air or water into the joints between the barriers or screens and the rod. Also, the space between the individual barriers or screens and the core has been filled with a compound or similar composition of good insulating properties. These measures have been considered necessary in order effectively to prevent the penetration of water into the joints between the barriers or screens and the rod.
Further known features concerning the assembly and selection of the insulating material for high-tension composite insulators are almost all concerned with the question of sealing the rod against environmental influences by means of the jacket surrounding it.
German accepted patent specification No. 12 96 341 describes the formation of the barrier or screen materials from a mixture of a cycloaliphatic epoxy resin or an unsaturated polyester resin with a suitable hardener and with aluminum oxide trihydrate as filler. A molding resin composition is selected as the core and this preferably consists of a mixture of an epoxy resin based bisphenol A with a suitable hardener and a filler, for example, quartz flour. The core is not reinforced with fibres and has no great mechanical strength. Moreover, there is a serious danger of inadequate insulation in the joint between the barrier or screen material and the subsequently cast-in core because, as the core is the last unit of the component and passes from the liquid into the solid phase, it tends to shrink away from the already solid material, centrally towards its axis.
In U.S. Pat. No. 3,898,372 a composite insulator is described in which prefabricated insulating barriers or screens having a bore diameter smaller than the diameter of the rod are pushed onto a resin-bonded glass-fibre rod, the joint between the screens and the glass-fibre rod being filled with an insulating grease. The sealing of the joints to the external atmosphere is achieved in that the insulating barriers or screens are compressed onto the rod with an axial pressure, so that seals result between the joints of the individual barriers or screens and between the last barriers or screens and the metallic suspension fittings on the ends of the insulator. The barriers or screens themselves comprise an ethylene-propylene-polymer rubber which is filled with inorganic fillers and is stable to creepage current and weather. Polyester resins, bisphenol epoxy resins and cycloaliphatic epoxy resins are specified as materials for the glass-fibre rod.
The basis of the type of insulator just described is that the barrier or screen material must be weather-resistant and resistant to creepage current. However, as to the properties of the supporting core, it is only said that, apart from a high resistance to longitudinal insulation breakdown, it must have a high mechanical tensile strength. The assumption is that the glass-fibre rod is protected absolutely against external influences by the barriers or screens or the screen jacket surrounding it.
It has now been appreciated that the known composite insulators of this type do not possess the requisite electrical strength, especially as regards their long-term behavior, and this may be attributed especially to the fact that the sealing between the insulator core and the barriers or screens is not entirely satisfactory.
According to the present invention, therefore, a composite insulator comprises a rod with barriers or screens surrounding it and an intermediate layer between the rod and the barriers or screens; the rod is of a non-saponifiable resin reinforced with fibre-glass of low alkali content; the barriers or screens are of a moisture-repellent, non-saponifiable polymer containing a filler and the intermediate layer is of a moisture-repellent, non-saponifiable polymer.